This invention relates to ring communications networks and, more particularly, to an arrangement whereby a passive line concentrator can be connected in a managed token ring network.
A token ring network is a well defined and standardized type of network arranged so that computers connected to the network may communicate with each other. Such a network includes a trunk which forms part of a normally closed ring and further includes at least one token ring concentrator having a number of input/output ports through which, by means of relay switching, computers may be inserted into the network. A standard governing token ring networks is the IEEE Standard 802.5, wherein each of the input/output ports is referred to as a trunk coupling unit. According to this standard, insertion of a computer into the ring is controlled by the computer itself, with the mechanism for effecting the insertion or bypass of the computer residing in the respective input/output port of the concentrator. Each computer is connected to a respective input/output port of the concentrator via a medium interface cable (MIC) and the computer exercises control of the insertion/bypass mechanism via the MIC using a phantom circuit technique. The phantom circuit impresses a DC voltage on the MIC. This DC voltage is transparent to the passage of computer-transmitted symbols, hence the name "phantom". The impressed voltage is used within the input/output port of the concentrator to effect the transfer of the switching action to cause the serial insertion of the computer in the ring. Cessation of the phantom drive causes a switching action which will bypass the computer and cause the computer to be put into a looped (wrapped) state. This loop may be used by the computer for off-line self-testing functions. The IEEE Standard 802.5 illustrates relay contacts for effecting the switching action to bypass/insert the computer. Typically, a latching relay is used because the impressed phantom voltage, and the resulting phantom current, is not sufficient to continuously power a relay.
Depending upon the particular network, there are two types of concentrators which may be utilized. The first type of concentrator may be referred to as a "passive" concentrator. This type of concentrator is used in a network which is not managed. Thus, there is no provision in this type of network for fault detection or reconfiguration of the network in the event of a fault. Each passive concentrator merely includes a plurality of input/output ports for connection to computers, as well as a ring-in port and a ring-out port for interfacing with a trunk cable connected between adjacent concentrators in the ring. Thus, the passive concentrator is unpowered and derives power for the switching functions from the computers to which it is connected.
The second type of concentrator may be termed an "active" concentrator. One use of an active concentrator is in a managed network. There are at least three aspects of network management. The first aspect is the collection of network statistics. The second aspect is fault tolerance wherein the network can be reconfigured in the event of a cable fault. The third aspect is topology management wherein unused portions may be removed from the ring to keep the ring as short as possible. An active concentrator will thus have some logic circuits and will have a source of power, but its input/output ports still rely on phantom current from a computer for inserting the computer in the ring.
There are circumstances where it would be desirable to use a passive concentrator in a managed network. In this way, the entire passive concentrator can be isolated in the event of a malfunction. It is therefore a primary object of this invention to provide an arrangement whereby a passive concentrator can be connected in a managed token ring network.